In wireless communication that transmits and receives digital data, limit of a wireless communication distance is determined by transmission power of radio waves, performance of an antenna used for transmission and reception, and a transfer rate. Since an increase in the transmission power is directly related to power consumption of a transmitter, there is naturally a limit. The performance of the antenna may be improved by using the Yagi-Uda antenna and the like, but as the performance of the antenna becomes higher, a structure thereof becomes bigger and complicated, so that the performance of the antenna that may be used is limited.
Also, restrictions are imposed on the transmission power by the Radio Law. Furthermore, depending on a frequency band of the radio waves, the transmission power including the performance of the antenna is regulated by the Radio Law. As a result, there is the limit of the transmission power and the performance of the antenna that may be used practically.
A direct sequence spread spectrum (DSSS), for example, is known as technology of realizing long-distance wireless communication escaping from such restriction (for example, refer to Non-Patent Document 1). The DSSS is the technology that enables highly sensitive reception while eliminating an influence of noise by multiplying a reception signal by a spread code to integrate. Extending integration time (that is, lowering a transfer rate) linearly increases the sensitivity, and in the global positioning system (GPS) adopting the DSSS, for example, the signal may be stably received even with an electric field intensity of −150 dBm or lower.
In the GPS, transmission radio waves are continuously transmitted at a stable phase. Therefore, stable phase synchronization is established even in a low signal to noise ratio (SNR) state by adopting a low-band phase locked loop (PLL) or delayed locked loop (DLL). If the phase is correct, weak signals may be detected by integrating and detecting the signals. In a case where a dedicated wireless frequency band is allocated like the GPS, continuous transmission over a long period of time becomes possible as described above, and stable reception may be performed even with the weak signal.
For example, there is a system that transmits information such as a sensor by using radio waves in the 920 Mz band. The 920 MHz band is a frequency band opened from July 2011 by the Ministry of Internal Affairs and Communications, and anyone can use this without a license. However, according to the provision (Association of Radio Industries and Businesses (ARIB) STD T-108), the maximum continuous transmission time is limited to four seconds. If the continuous transmission time is further shortened to, for example, 0.2 seconds, more channels may be allocated, and it is possible to perform transmission and reception with less crosstalk.